1. Field of this Invention
This invention relates to a liquid crystal (LC) compound, an LC composition and a liquid crystal display (LCD) device, and more particularly relates to a naphthalene compound with a terminal hydrogen atom, an LC composition including the compound and an LCD device including the liquid crystal composition.
2. Description of the Related Art
In a LCD device, classification based on an operating mode of LCs includes phase change (PC), twisted nematic (TN), in-plane switching (IPS), super twisted nematic (STN), optically compensated bend (OCB), electrically controlled birefringence (ECB), vertical alignment (VA) and so forth. Classification based on a driving mode includes a passive matrix (PM) and an active matrix (AM). The PM is further classified into static, multiplex and so forth, and the AM is further classified into a thin film transistor (TFT), a metal-insulator-metal (MIM) and so forth. Each of the LCD devices includes an LC composition with suitable properties. In order to improve the general properties of LCD devices, the LC compositions are required to have the following general properties:
General property of LCGeneral property of LCDNo.compositionsdevices1being thermally stablelong lifetime2being UV-stablelong lifetime3wide temperature range forwide temperature range fornematic phaseuse4low viscosityshort response time5low threshold voltagelow power consumption6high specific resistancehigh voltage holding ratioIn addition, the LC composition should allow UV to be applied in the LCD process. It is also desired that the LC composition can be injected into an LC cell in a short period.
The above properties 1-6 are all important to an LC composition used in an AM devices while only the properties 1-5 are important to that used in a PM device. Except the above properties, the optical anisotropy (Δn), the dielectric anisotropy (Δ∈), the elastic constants and so forth are also important. For a single LC compound cannot meet all the requirements, multiple LC compounds are mixed into an LC composition in use. Hence, good compatibility between LC compounds is required, especially in low-temperature conditions. Meanwhile, because recently LCD devices are miniaturized and the device driving voltage is lowered, LC compositions with low threshold voltages are required. According to H. J. Deuling, et al., Mol. Cryst. Liq. Cryst., 27 (1975) 81, the threshold voltage (Vth) can be expressed by the following equation:Vth=π[K/(∈0·Δ∈)]1/2 wherein K is the elastic constant of the composition and ∈0 is the dielectric constant of vacuum. According to the equation, the Vth value can be lowered in two ways, i.e., increasing the dielectric anisotropy (Δ∈) or decreasing the elastic constant. However, since controlling the elastic constant of an LC composition is difficult, LC compositions with large dielectric anisotropy are usually used, thus causing great development of LC compounds with large dielectric anisotropy.
Moreover, for the techniques of fabricating LCD substrates are better recently, the cell gap tends to be smaller, especially for the AM devices of TN mode, OCB mode or the like. The cell gap is the distance between the two substrates, being equal to the thickness of the LC composition. When the cell gap of a TN-mode device is small, the response time is short and a reverse domain is difficult to form. When the cell gap of an OCB-mode device is small, the transfer time from splay alignment to bend alignment becomes short, and the response time, i.e., the time needed for the alignment of the LC molecules to change in response to the change of the applied voltage, is also short.
In addition, the product (Δn×d) of the optical anisotropy (Δn) and the cell gap (d) is constant for the above modes. In such a case, the optical anisotropy should be large if the cell gap is small, so that a composition with large optical anisotropy is required. Hence, LC compounds with large optical anisotropy are also greatly developed, as described in the following patent documents: Japan Patent Publication No. Hei 4-504571, Japan Patent Publication No. 2001-26560, GB 2227019A, GB 2271771A, DE 10251017A and WO 2004/029015 pamphlet.